Proton pulsed

Gutman M 1986 Application of the laser-induced proton pulse for measuring the protonation rate constants of specific sites on proteins and membranes Methods Enzymol. 127 522-38... 

Figure 5.14 Pulse sequence for selective indirecty-spectroscopy. The three proton pulses at the center of the evolution period flip attached protons selectively, resulting in decoupling between distant and attached protons. (Reprinted from J. Magn. Reson. 60, V. Rutar, et ai, 333, copyright (1984), with permission from Academic Press, Inc.)...

The selective INEPT experiment also requires a separated proton resonance for excitation and also uses rectangular soft pulses (Bax and Freeman, 1982). Magnetization is selectively transferred by the soft proton pulses to the corresponding carbons to which the protons are coupled. 

Now we use a normal proton pulse to excite the solute molecule spectrum (d) shows the result of the presaturation experiment carried out on the H20/ D20 solution of model compound 1. A residual H20/H0D signal can be ob-... 

Fig. 10.14. Gradient-enhanced HMQC pulse sequence described in 1991 by Hurd and John derived from the earlier non-gradient experiment of Bax and Subramanian. For 1H-13C heteronuclear shift correlation, the gradient ratio, G1 G2 G3 should be 2 2 1 or a comparable ratio. The pulses sequence creates heteronuclear multiple quantum of orders zero and two with the application of the 90° 13C pulse. The multiple quantum coherence evolves during the first half of ti. The 180° proton pulse midway through the evolution period decouples proton chemical shift evolution and interchanges the zero and double quantum coherence terms. Antiphase proton magnetization is created by the second 90° 13C pulse that is refocused during the interval A prior to detection and the application of broadband X-decoupling.

One way to handle this problem, within the framework of the CPMG scheme, is to add proton pulses at judiciously chosen points in the train of the carbon 7r-pulses [42, 43]. An example of such a sequence is given in fig. 4(a). An important issue when setting up experiments of this type is the duration 8 between the pulses. On the one hand, it should be small compared to (l/2)Jis [43]. On the other hand, it should be much longer than the relevant pulse widths. It is difficult to simultaneously fulfill both these requirements rigorously with typical high-resolution equipment, and some compromise has to be settled on. 

The DEPT sequence (distortion enhancement by polarization transfer) has developed into the preferred procedure for determining the number of protons directly attached to the individual 13C nucleus. The DEPT experiment can be done in a reasonable time and on small samples in fact it is several times more sensitive than the usual 13C procedure. DEPT is now routine in many laboratories and is widely used in the Student Exercises in this textbook. The novel feature in the DEPT sequence is a variable proton pulse angle 9 (see Figure 4.11) that is set at 90° for one subspectrum, and 135° for the other separate experiment. 

Our simple 2-D experiment is actually a very important experiment sometimes simply called COSY (Correlation SpectroscopY), and which we will call COSY for the time being in order to clearly indicate what is being correlated. The pulse sequence for H — H COSY is none other than the one we have already described above in Figure 5.3 two tt/2 proton pulses separated by the required evolution period, q, which is systematically incremented, and the acquisition period, t2. 

The pulse sequence for ICP experiments appears simple a 90° proton pulse is followed immediately by a spin lock radio-frequency (rf) field of strength B that is phase shifted by 90° relative to the first pulse. By a spin-lock field is meant a strong rf field B that is on resonance with the given nucleus it keeps magnetization in a spin-locked orientation parallel to the B direction where the decay of magnetization is governed by T p. At present the strong continuous B field is replaced by multipulse sequences that are well known from other spin-lock experiments such as TOCSY, ROESY etc. Simultaneously,... 

The selectivity of INEPT can be achieved in the most straightforward way, as suggested by Bax and coworkers301 302, by replacing all the hard non-selective proton pulses of INEPT with selective ( soft ) pulses (this pulse sequence is sometimes denoted as SPINEPT). To retain full sensitivity, the selective pulses must cover the selected multiplet and its 29Si satellites, i.e. the excitation band should be approximately equal to the width of the multiplet plus the value of /(29Si—XH) to be used for polarization transfer. For the choice of suitable selective pulses, see Reference 135. 

FIGURE 51. Measurement of 29Si spin-lattice relaxation time in 1,1,3,3-tetramethyldisilazane using conventional inversion-recovery (top, measuring time 6.5 h, T = 37.6 1.4 s) and INEPT enhanced version (bottom, measuring time 45 min, T = 38.1 0.9 s) with the phase of penultimate proton pulse +y. Reproduced by permission of Academic Press from Reference 357... 

There are two important consequences of this method. Both result from the fact that the magnetization that is being observed (13C antiphase doublet) arises from H magnetization that is rotated from its equilibrium state along the z axis by the first 90° proton pulse. The first consequence arises because the lH population difference at equilibrium (sometimes called polarization ) is four times the carbon population difference at equilibrium. This results from the larger energy separation between the a and states for protons ... 

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